Update jordan/icinga2 Docker tag to v2.13.7

This MR contains the following updates:

Package	Type	Update	Change
jordan/icinga2	final	patch	2.13.6 -> 2.13.7

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webserver/docker_icinga/templates/Dockerfile

-FROM jordan/icinga2:2.13.6
+FROM jordan/icinga2:2.13.7
 
 # Install additional Packages
 RUN apt-get update \